For many years asbestos fibers were the preferred principal ingredient in high performance friction materials. Asbestos is not only plentiful and inexpensive but, more importantly, it possesses the properties known to be desirable for effective braking. Specifically, asbestos has varying fiber length, porosity, compactability, excellent fibrillation, and good thermal qualities. The asbestos mix for friction materials generally comprised 70% asbestos and only 30% fillers, such as chalk, rubber, and limestone.
Unfortunately, asbestos is now regarded as a carcinogen and is not generally utilized in the United States for the manufacture of friction materials. A variety of materials have been and are being used in place of asbestos in friction materials, including aramids, glass wool, steel wool and fibers, ceramic fibers, and carbon fibers.
Due to the relatively high cost of man-made fibers used as alternatives to asbestos, current friction materials have only 1 to 15% of the man-made alternatives to asbestos and between 85% to 99% fillers.
Almost all brake friction materials have these component materials: (1) fibers, (2) fillers, (3) binding resins, (4) friction particles, and (5) friction modifiers.
These components are used in three different types of brake friction materials which are generally known as (1) organic, (2) semi-metallic, and (3) metallic friction materials. The components of these three types of friction materials are shown below:
______________________________________ Example A SEMI- ORGANIC METALLIC METALLIC ______________________________________ FIBERS asbestos aramids and/ steel wool and/or glass or glass fibers; and/or fibers; cotton carbon; ceramic carbon; ceramic FILLERS chalk and/or barium sulfates barium sulfates limestone; and/or cashew and/or rubber; baryte particles; iron iron powders calcines powders INDERS phenolic or phenolic or phenolics cashew cashew modified modified phenolics phenolics FRICTION alumina silicates; iron powders; PARTICLES oxides; oxides; zinc; oxides; silicates brass; copper brass; copper FRICTION antimony graphites; graphites MODIFIERS and/or rubber graphite ______________________________________
The end uses of the three different types of brake friction materials are summarized in Example B:
______________________________________ Example B END USES BRAKE FRICTION MATERIALS ______________________________________ Passenger cars: Organic; Non-metallic; Semi-metallic; and Metallic. Racing Cars*: Organic (asbestos); and Metallic. Industrial: Organic and Inorganic (asbestos and Non-asbestos). Heavy Duty: Semi-metallic and Metallic. (Trucks and Off-road vehicles; Military) Aircraft: Sintered metallic; Carbon/carbon fiber; Semi-metallic. ______________________________________ *Some racing cars use carbon/carbon fiber and sintered composites.
High performance brakes as used in racing cars, for example, need to tolerate sustained high temperatures, and use metallic fibers, whereas lower performance brakes as used, for example, in industrial applications use semi-metallic materials such as aramid fibers for longer wear. Aramid fibers are known to promote improved wear.
In the United States, manufacturers of race car brakes are still relying on asbestos-containing materials. With the exception of the carbon/carbon fiber materials, the United States has not started producing non-asbestos friction materials for racing until very recently, and with only a limited degree of success.
In addition to the health stigma, asbestos has the disadvantage of causing high wear on rubbing surfaces, especially under racing conditions. There exists a need for viable alternatives to asbestos.
Efforts to find a satisfactory replacement for asbestos are illustrated in the following patents:
__________________________________________________________________________ PATENT NO. ISSUE DATE INVENTOR TITLE __________________________________________________________________________ 4,145,223 Mar. 20, 1979 Iwata FRICTIONAL MATERIAL FOR BRAKES 4,374,211 Feb. 15, 1983 Gallagher ARAMID CONTAINING FRICTION MATERIALS 4,476,256 Oct. 9, 1984 Hamermesh FRICTION MATERIAL FOR BRAKE LININGS AND THE LIKE 4,656,203 Apr. 7, 1987 Parker FRICTION MATERIALS AND THEIR MANUFACTURE 4,722,949 Feb. 2, 1988 Horiguchi NON-ASBESTOS FRICTION COMPOSITION CAPABLE OF MANUFACTURE IN DRY PROCESS __________________________________________________________________________
The newer metallic materials, such as disclosed in Iwata, Hamermesh, and Parker are not considered adequate in racing applications because these materials generate very high heat under sustained heavy braking and this causes premature material oxidation and consequent disintegration of the friction material.
Gallagher substitutes aramid polymer pulp fiber, a fibrous reinforcing material, and a resinous binder for asbestos and metallic fibers, but the aramid fibers are not effective in racing car brakes and other high performance applications where the friction material is subjected to sustained high friction and temperatures in excess of 1,000.degree. F.